I have a DS3 (bios F7) and E6400. I've been messing with overclocking too, and this happened once, where I'd do something wrong, and change the settings back to my last stable result, or try some other settings, and then find that it had reset to (almost) stock settings instead. I'm not sure what caused it, and it only happened once, so I just hope it doesn't happen anymore.
If I remember right, the only thing it really changed was the FSB frequency back to the stock 266. As far as I remember, it left my voltages (which were a big part of my problem ) intact.